34 Edmund B. Wilson 



the existence of a quantitative difference in some cases raises the 

 question whether it is not the result or expression of some more 

 deeply lying nuclear difference which may still be present in those 

 cases where no quantitative difference exists. I find it altogether 

 incredible that two animals as nearly related as Nezara and 

 Euschistus should differ fundamentally in the relation of the 

 chromosomes to sex-production; and if there is any reason to 

 conclude that sex-determination is effected by the idiochromo- 

 somes (or by the combination of which they form a part) in the 

 case where they are visibly different, I cannot avoid the belief 

 that this conclusion applies with equal reason to the case in which 

 they appear to the eye alike in all the spermatozoa. It therefore 

 seems to me an admissible hypothesis that a physiological or 

 functional factor may be present that differentiates the spermat- 

 ozoa into male-producing and female-producing forms irrespec- 

 tive of the size of the differential chromosomes; and further, 

 that the morphological difference that has arisen in some forms 

 may have been a consequence of such an antecedent functional 

 difference. If we could assume for instance that the differential 

 chromosome-pair in the male includes a more active and a less 

 active member (the latter having in many cases become reduced 

 in size or even having entirely disappeared) the suggestion might 

 be greatly extended in application. Under this assumption the 

 facts might receive a general formulation in the statement that 

 the association of two more active chromosomes of this class 

 produces a female, while the association of a more active and a 

 less active one (or the absence of the latter, as in case of the hetero- 

 tropic chromosome) produces a male. Reduction of the less 

 active member to form a small idiochromosome would introduce 

 a quantitative difference of chromatin as well as a qualitative one. 

 Its complete disappearance in the male, leaving only the active 

 member as the heterotropic chromosome, would reduce the 

 difference to a merely quantitative one. The assumption of 

 such a physiological difference is admittedly a purely specula- 

 tive construction, and may seem a priori very improbable. But 

 from the a priori point of view it would seem equally improbable 

 that a morphological dimorphism of the spermatozoa, affecting 



